Vintage early 60’s LECTROLAB tube amp We have owned a bunch of Lectrolabs over time and have two left… This one is a just a great sounding small room or recording amp… 2 X EL84 in the power section into a 12″ Alnico magnet speaker… 4 inputs so you can jump them and blend the two channels… This has been gone over by American Music here in Seattle and is ready to play… Speaker is vintage ROLA low powered Alnico 12-15 watts…check out VG article this month on alternative speakers… Tubes are new power tubes with a mix of new and possibly original preamp tubes… Great raw sounding EL-84 amp…way cooler than a new Chinese AC4

Way cooler than a lot of things, my friend…

Example 2 was a Craigslist posting brought to our attention by Jim Kachel, a Lectrolab S600 owner. I thought it was possible that examples 1 and 2 were the same amp until Papa (who now owns example 2) set us straight – his comment is below.

And finally, Phil, the third S500 owner discovered here, has sent us a schematic. Thanks to all you!

Now that we have a schematic we can comment on the design:

1st gain stage is a 12AX7, standard Fender-type design but without a cathode bypass capacitor, so lower gain. Jacks 1 and 2 feed one triode, 3 and 4 the other triode. So two preamp channels, identical except for the grid leak resistors. 1/2 share a 390K, 3/4 a 2.7M. This means the second channel, which is input jacks 3 and 4, should have more gain than the first channel.

After the first stage the signal passes through a simple tone circuit (treble attenuation) and volume control to the second stage. I think the schematic is missing a ground symbol that would connect to the junction of the tone and volume controls as well as the second stage cathode resistor.

The second gain stage is another standard 12AX7 triode amplifier, again with no cathode bypass capacitor.

That uses up 1-1/2 12AX7’s so far. The second half of the second tube is a standard concertina phase splitter which feeds…

a 6BQ5 (EL84) push-pull output pair, the power amplifier. Again, a typical arrangement with 100K grid leak resistors, no grid stop resistors. The screens are fed through a single 680 ohm resistor from B+, which according to penciled notes on the schematic is 307 volts. The screen resistor drops that to 296 volts on the screens.

Output tube bias is fixed, at -10.7 volts according to pencil marks, and derived from a voltage divider on the B+ winding of the power transformer. Most 2-EL84 amps are cathode-biased. Initially this because it was cheaper due to fewer components, and subsequently because most of these designs now slavishly copy AC15’s. Fender Blues Jr. is a current 2-EL84 amp that has a fixed bias. Neither approach is inherently better or worse, they both sound fine, and tone differences between them are subtle.

There is no negative feedback loop (NFB) around the power amplifier. This is not subtle. NFB reduces both gain and distortion. The Fender “sparkly clean” sound is dependent on NFB, and the Vox “grit” relies on its absence. Other design choices are at play in both amps, but this is a significant factor in tone.

One side of the 6.3 volt heater supply is “lifted” above ground by another voltage divider shown as “A” on the schematic. I estimate this lift at about 22 volts DC (47K/390K*182V), and it is there to reduce hum from the AC heater wiring.

All in all, this is a fine, respectable circuit that should last a few lifetimes and is in the same class as a Vox AC15 or Matchless Spitfire or any number of 2-EL84 amps. Well… one difference is that you can buy it (if you can find it) for under $300! And a caveat here is that I don’t know how the transformers, which are important components, compare with other amps. All the blather you read about resistor and capacitor types is a just that, any changes (except for replacing bad ones) in that area is at best a very, very thin layer of icing on the cake. But it’s religion for some folks.

Can you tweak this amp? Sure! If it’s like every other Lectrolab I’ve seen it’s not hard to work on it, assuming you know how to work on amps. If you don’t, then send it to someone who does. Otherwise, the people who rely on you being alive may come to me for damages! Choices are limitless but some mods might be:

Add a three prong AC plug. Always a good idea.

Goose the gain by adding bypass capacitors to the first and/or second stages

Add a resistor for each 6BQ5, replacing the one that feeds both. These resistors can be increased in value to provide greater protection to the tubes, and should be flame resistant in either case.

Add a grid stop resistor to the output tubes to increase high frequency stability. 10k will probably do it.

Possibly get rid of C5, which shunts high frequencies around R14 in an effort to improve HF stability. It may or may not be required.

Replace R26 with a resistor and trimpot that will give you come control of the bias voltage.

Go crazy – rewire the two channels of stage one to be in series, thus adding another gain stage to the circuit and creating a metal-worthy gain monster. You’ll need another volume control in there.

But seriously, folks. None of this is necessary. There is probably enough gain in the amp to drive it into some very sweet power tube distortion, and it should just fine if you back off the volume and play it clean. If the amp is working then it’s been working for over 40 years. If the electrolytics are good, the amp should sound great.

Hi Gary, thanks for the comment. I found those pictures on eBay, so if you won the auction at $285 on September 15th, then these pics show your amp. Congratulations, you have the only S500 I have ever seen! Please let us know how it sounds – send us some sound clips!

They are not the same amp. #2 is sitting next to me here – it’s not on Craigslist anymore! It has an earlier (40th week of 1962 by the date code) ROLA alnico replacement speaker. The splice on my new amp is not visible in the rear photo of amp1. Amp appears to date from 65-66.

Schematic is on the back of the rear panel. I’ll send you a photo when I get a hi-res shot.

Thanks for the info, Papa, and welcome to the Lectrolab community! It will be GREAT to receive and post that schematic, thank you for that. It doesn’t need to be a perfect pic, as long as it’s legible.

I am a 16 year old male and my grandmother had given me a Lectrolab S500 guitar amplifier when i was really young it is in good condition but the tubes need to be replaced. It has the schematic papers inside where it should be and i was wondering where i could buy some so i can sell it. I need the extra money so i can get new equipment but at the same time i don’t want to lose an old friend

David – my advice to you is to keep the amp and figure out another way to get new equipment. There are thousands upon thousands of us older folks who each have many stories about the gear we got rid of when we were kids, and we regret it later. Whatever new stuff you want, there is prppbably lots of it out there. How many S500s are out there? Very, very few. And it’s a good amp when in working condition. How many are out there that your grandmother gave you? ONE! There must be family history in that amp, was it your grandfather’s? You can’t buy that later, no matter how rich you get. Trust me, you won’t regret keeping it 20 years from now.

You can buy new tubes in many places, google “guitar amp tubes” and then compare prices. BUT, unless you’ve already tried other tubes you don’t know that the tubes are a problem. Is there a music store in you town? They can probably steer you to a local amp repairperson who can look it over and get it into shape. Good luck! And send us some pictures!

HI all, cool I found this site,I am also the very PROUD owner of the S500 ,I am very broke and would not sell this amp if I needed food,to the youngster who wants to sell this amp for something new ,as the other poster stated PLEASE DON’T at 16 you probably don’t have the experience to truly understand what this amp can DO (not a dis). The sound is blues perfection crunch with warmth and articulation in the picking attack,as you turn up it starts to roar,a good thing ,with a 25DB clean boost it just SINGS (that wont hurt it I hope, let me know) If you are lucky enough to find one BUY IT (I will post a sound bite when the D@%$ next door moves next week.
Thanks all and let me know about the boost if anyone knows.

I appreciate the sentiment you all seem to share. A friend of mine recently brought me a Lectrolab S500 amp *with* a matching 12” external speaker cabinet. I fired it up and it sounds awesome, although channel 2 was a bit weak and the power cord is somewhat dry-rotted, so I’m sure it needs the wiring updates talked about above. There was an audible hum also, but that seemed to be more related to grounding than anything as it changed when I touched the metal on my jack. I do play and would love to buy it for myself, but I honestly don’t have any room for more gear nor time for another project. Any interest here? This seems like the place for folks in the know.
Blessings,
Pastor John